1. Technical Field
The present invention relates to methods of manufacturing reduced-warp nitride substrates for semiconductors, and to nitride semiconductor substrates manufactured by the method.
2. Description of the Related Art
Substrates on which semiconductor devices are fabricated are round wafers, and given that the devices are fabricated on the front surface of the substrates by such methods as photolithography, doping, diffusion, and vapor deposition including chemical vapor deposition (CVD), the front surface must be flat, with minimal warp. When fabricating semiconductor devices onto silicon and onto gallium arsenide in particular as substrates, Si and GaAs wafers with minimal warp, polished to an optically smooth, mirror finish are employed.
Sapphire wafers are used as the substrates for blue light emitting diodes in which indium gallium nitride is the light-emitting layer. InGaN/GaN-based LEDs formed onto sapphire substrates have performed well and are dependable. The sufficiently moderate cost of sapphire substrates has meant that InGaN-based LEDs can be made at low-cost.
Nevertheless, there are drawbacks to sapphire. For one, with sapphire being an insulator, rather than attaching n electrodes to the bottom, a GaN layer onto the surface of which the n electrodes are attached is applied, thus requiring excess area. Another is that since sapphire does not cleave, it cannot be rent into chips along natural cleavages. And because it is GaN and InGaN that are grown onto the heterosubstrate there is misfit, which leads to heavy defects.
Under the circumstances, then, it is desirable that GaN itself be the substrate. GaN substrates have become producible by depositing a thick GaN film onto a heterosubstrate base using vapor-phase deposition and removing the base to create a GaN freestanding layer. And in terms of size, 50-mm diameter Substrates—long-awaited—have also become possible.
Vapor-phase grown GaN-crystal wafers are, however, used as-grown for epitaxial deposition substrates. In the front side of GaN substrates that have been vapor-phase deposited and nothing more roughness is considerable and warp is serious; growing GaN and InGaN onto such substrates will not necessarily lead to a lowering of defects over the situation with sapphire substrates. And LEDs created experimentally on as-grown GaN substrates certainly do not perform better than LEDs manufactured on sapphire.
Because the formation of semiconductor devices onto GaN substrates is by photolithography, flat, mirror-finish wafers with minimal warp are desired as the substrates. Polishing and etching technology is necessary to render the surface of a wafer optically smooth. Polishing and etching technologies have already been established for fully developed semiconductor substrates such as Si and GaAs. Si and GaAs crystal can be grown by gradually solidifying a melt, as in the Czochralski method or the Bridgeman method. Since long, columnar ingots with few dislocations can be produced by growing from the liquid phase, the ingots are sliced with an internal-diameter saw to produce wafers. This means that warp is minimal from the start.
With GaN on the other hand, growth, being impossible from the liquid phase, is by means of vapor-phase deposition. Furthermore, what form optimal polishing and etching methods should take is still not understood. If GaN is to be hetero-deposited onto crystal of a different kind, such as has three-fold symmetry, the growth will necessarily be c-axis oriented. The surfaces are a (0001) plane and a (000 1) plane. Because GaN crystal does not have reverse symmetry, the (0001) and (000 1) planes are not crystallographically equivalent. The (0001) face is one in which gallium atoms range in lines globally over the episurface, and the (000 1) face is one in which nitrogen atoms range in lines globally over the episurface.
The former can be referred to as the (0001) Ga face, or simply the Ga face; the latter, as the (000 1) N face, or simply the N face. Physiochemically the Ga face is extremely unyielding and rugged, and is not dissolved by chemical agents. The N face is also physiochemically robust, but is corroded by certain types of strong acids and alkalis. GaN crystal has such asymmetry.
When GaN is grown onto a base substrate, the front side and back side become either the Ga face or the N face. Depending on how the base substrate is selected, the front side can be made the Ga face or the N face. The back side then becomes the face with the opposite polarity.
For the sake of simplicity, a case in which the front side is the (0001) Ga face, and the back side is the (000 1) N face will be considered. The same statements can be made, and the same design features implemented in the opposite situation as well.
Since the subject of the present invention is warp, to begin with a definition of warp will be given. Warp can be expressed as radius of curvature, or curvature. These are exact expressions and can be given locally. Even in situations in which the warp is complex and the substrate has heavy roughness, exact warp can be expressed using a local curvature expression. For example, warp with a saddle point and cylindrical-lens-like warp can also be expressed.
But with uniform buckling in round wafers, warp is often represented by a simpler expression. If the roughness is uniform the wafer is measured taking the height H to the planar face from the surface of the center area in the convexity, according to which a value for the warp is given. This is intuitive, and facilitates measurement. The absolute value is determined by this warp measurement.
The sign of the warp will be given by its orientation. This definition is indicated in FIG. 1. Warp curving outward along the front side will be termed positive (H>0); warp curving inward along the front side will be termed negative (H<0).
In situations in which long monocrystal ingots with few dislocations can be produced—such as is the case with Si and GaAs—since the ingots are sliced with an internal-diameter saw or a wire saw, warp is slight from the start. To produce GaN crystal, however, with growth from the liquid phase being impossible, vapor-phase growth is carried out. Because rendering GaN crystal is by heteroepitaxy onto a heterosubstrate that differs from GaN in thermal expansion coefficient, and then removal of the heterosubstrate, considerable warp appears in GaN crystal. This problem is due not only to the difference in thermal expansion coefficient, but also to the many dislocations that come about because the base substrate and the overlying film are different materials. The dislocations give rise to irregular stresses, which due to the volume of dislocations is why warp comes about.
As-grown, platelike, 20-50 mm diameter GaN crystal from which the base substrate has been removed has a warp of from ±40 μm to as much as ±100 μm, although the value will differ depending on the type and crystal-plane orientation of the base substrate, and on the vapor-phase deposition parameters.
With the warp in a GaN wafer substrate being that extensive, in a situation in which a photolithographic resist on the wafer is to be exposed its dimensions will be thrown out of balance. Thus the warp must be extensively reduced. Warp in Si and GaAs wafers also has to be lessened, but with GaN there is a special reason why warp has to be reduced. Since GaN is transparent, when the wafer is set on a susceptor with a built-in heater and heated, not much of radiant heat from the heater heating the GaN crystal occurs. Seeing as how thermal conduction from the susceptor is the principal heat-transmission means, the back side of the GaN crystal desirably is flat, with its entire surface in contact with the susceptor without gaps.
Instances of the above outward-curving (positive warp, H>0) mean that the wafer center portion comes apart from the susceptor. Such cases are still the better, because the thermal conduction is from the peripheral margin heading toward the center. Oppositely, in instances of the above inward-curving (negative warp, H<0), with only the center contacting susceptor the wafer ends up turning, leading to positional instability. Not only that, but source-material gases circle to the back side through the encompassing, lifted-up area, causing thin-film growth or etching to occur on the backside of the substrate also. Consequently, negative warp is even less suited to semiconductor fabrication needs than positive warp.
Because as-grown GaN crystal has a warp H of from ±40 μm to ±100 μm, the number one objective is to decrease the warp to be within a +30 μm to −20 μm range.
More advantageously, the warp should be decreased to within a +20 μm to −10 μm range.
Furthermore, if possible, bringing the warp to within +10 μm to −5 μm would even better meet fabrication needs.
There are any number of examples of devising a crystal growth method to minimize warp in the products. These may be grossly bifurcated into those that reduce warp by lateral overgrowth of the GaN to alleviate vertically oriented stress and reduce internal stress, and those that grow two layers having competing actions and eliminate warp by the balance between the actions. Every one of these is a way of attempting to reduce, via the deposition parameters, warp in crystal being grown; they are not ways of attempting to reduce warp in crystal already produced.
Japanese Unexamined Pat. App. Pub. No. H11-186178 addresses the problem of incidents of warp and cracking in GaN crystal that due to the difference in the coefficients of thermal expansion of Si and GaN occur when a GaN film is grown onto an Si substrate to create a GaN/Si composite substrate.
This reference relates that to prevent warp and cracking from occurring in GaN crystal, stripes of SiO2 film are formed onto an Si substrate, and when GaN film is grown onto the substrate, atop the SiO2 growth of GaN does not initially occur, thereby alleviating stress and reducing warp in the GaN/Si composite substrate. This substrate is not an independent film of GaN, but rather a composite substrate in which a thin GaN layer on the order of 10 μm is provided on an Si base, so that internal stress in the GaN layer can be reduced by having the SiO2 intervene.
Japanese Unexamined Pat. App. Pub. No. 2002-208757 concerns manufacturing nitride semiconductor substrates of satisfactory crystallinity, by employing lateral overgrowth and, to keep warp under control, dispersing throughout the substrate overall the coalescence boundaries, where defects concentrate.
Japanese Unexamined Pat. App. Pub. No. 2002-335049 proposes a deposition method that by reducing dislocations by means of lateral overgrowth to diminish stress, also reduces warp.
Japanese Unexamined Pat. App. Pub. No. 2002-270528 proposes a deposition method in which reducing dislocations by means of lateral overgrowth to reduce stress keeps warp from occurring.
Japanese Unexamined Pat. App. Pub. No. 2002-228798 exploits Si crystal not as a semiconductor but as a mirror. The goal is to create concave or convex mirror surfaces from Si crystal. To get Si crystal to possess a desired curvature, it must be deformed. To do so, a thin film of diamond is built up on an Si substrate, and the Si substrate is deformed by the stress between the diamond thin film/Si substrate. In other words, the original planar article is forcibly buckled to lend it a concave or convex mirror surface. The reference states that Si can be buckled into a curvature of choice depending on the diamond formation parameters.
Japanese Unexamined Pat. App. Pub. No.2003-179022 addresses the problem that after forming semiconductor devices onto a large-caliber Si wafer, the wafer is back-side ground and the back side is mechanically planed to reduce the wafer to a desired thickness, but a processing distortion layer is formed, producing a warp of 800 μm, and etching away the layer takes too much time. This reference states that, given the realization that the processing distortion layer on the Si wafer back side is amorphous, warp is eliminated by exposing the Si back side for 5 seconds with light from a halogen lamp to momentarily heat the wafer to 600-700° C. and convert the processing distortion layer from an amorphous to a crystalline state. Thus this is an example not of ridding the wafer of the processing distortion layer, but eliminating warp in the wafer by qualitatively transforming the layer.
Inasmuch as nitride semiconductor is chiefly produced using vapor-phase deposition to build up a thin film onto a heterosubstrate and removing the base substrate, with dislocations due to the difference in thermal expansion coefficients and the mismatching lattice constants occurring at a high density, warp is serious. Although methodologies for diminishing warp by devising growth methods to diminish internal stress have been variously proposed, they are yet insufficient.
Even with such methodologies, manufacturing nitride semiconductor crystal of large film thickness and large diameter means the dislocations and warp will be considerable, and when the base substrate is removed the crystal often ends up cracking. Even if the crystal does not crack, the warp will be large, reaching ±40 μm to as much as ±100 μm.